I personally used this online quiz when I first suspected I had ADHD. After completing, my family doctor had me fill out a diagnostic self-assessment. Sometimes, you might be referred to a Psychiatrist for a diagnosis.
Self-Test: Could you have ADD/ADHD?

Suspect your child has ADD/ADHD? The Snap IV is often used by clinicians to diagnose ADD or ADHD. Print off the form and bring to your primary health provider to discuss the results and next steps.

Check out this list of common symptoms adults with ADHD experience.